Examples of a high-strength cold-rolled steel sheet having a high r value include an aluminum-killed steel sheet containing phosphorus added thereto (see, e.g., Japanese Patent Publication No. 20733/1984), and an ultra-low carbon steel sheet containing titanium and niobium and, added thereto, phosphorus (see, e.g., Japanese Patent Publicatation No. 47328/1985) and a Ti-containing super-low carbon steel sheet containing P and Cu in combination (see Japanese Laid-Open Patent Application No. 61-15948). However, the tensile strength of these high-strength steel sheets is 40 to 45 kgf/mm.sup.2 or less at the most. Therefore, they do not meet the above-described new demands for recent cold-rolled steel sheets.
There is an ever-increasing demand from users with respect to an increase in the performance of the material for a recent cold-rolled steel sheet having high workability. That is, in addition to an increase in the demand for parts having a complicated shape requiring high work deformation, there is an ever-increasing need of a decrease in the weight of the parts through an increase in the strength of the parts and a decrease in the thickness of the steel sheet. Further, in recent years, on the users' side of the steel sheet as well, there is an increasing need of reducing cost thought a reduction in the number of steps of working for deformation as much as possible. Therefore, conventional steel sheets do not meet at all the above-described users' demands.
The tensile strength of a high-strength steel sheet having a high r value produced in the prior art is 45 kgf/mm.sup.2 at the most. It is a common knowledge in the art that in general the addition of various reinforcing elements for the purpose of increasing the strength of the steel sheet brings about a lowering in the r value with an increase in the strength thereof and therefore makes it impossible to attain a high r value in the case of a high-strength steel sheet.
The present inventors have developed a novel heat-treatment-hardenable cold-rolled steel sheet having a high r value even when the tensile strength is 45 kgf/mm.sup.2 or more, and a process for manufacturing the same.